The present invention relates to a VDSL transmission system of DMT(Discrete Multi-Tone) modulation/demodulation scheme, and more particularly, to a time-domain equalizer of cascaded dual filter in a VDSL transmission system which is adapted to maintain interoperability with an ADSL transmission system.
FIG. 1 illustrates a functional block diagram showing a transmitter and a receiver of a DMT transmission system. Core technologies in the DMT transmission system is the RS coding, the TCM(Trellis Coded Modulation), the bit loading, the TEQ(Time-domain Equalizer), the FEQ(Frequency-domain Equalizer, and modulation and demodulation by using the IFFT(Inverse Fast Fourier Transform) and the FFT.
The operation of the modulation/demodulation device(a modem) using the Fourier Transform will be described.
A frequency domain DMT symbol X=[X0, X1, - - - , XNxe2x88x921]T has N complex QAM symbols, wherein each of the QAM symbol forms a subchannel to form total N subchannels. An (n)th QAM symbol Xn[an (n)th subchannel symbol] is modulated with a digital carrier Pn=[1, ejwn, ej2wn, - - - , ej(Nxe2x88x921)wn]T having a length N and a frequency xcfx89n=2xcfx80n/N. Therefore, (m)th DMT symbol may be represented as Xm=[Xm,0, Xm,1, - - - , Xm,Nxe2x88x921]T, wherein, if an N number of QAM symbols are modulated with digital carrier and Xm,k value at time k is calculated, Xm,k=Xm,0*P0,k+Xm,1*P1,k+- - - +Xm,Nxe2x88x921*PNxe2x88x921,k is obtained, where Pn,k=ejkwn of a carrier Pn at time k. That is, it can be known that Xm,k is a (k)th signal obtained by subjecting Xm to Fourier transform. Accordingly, a DMT modem can modulate N QAM symbols(N subchannel symbols) at a time by using N point inverse Fourier transform. Alikely, transmitted symbols can be demodulated by using the N point Fourier transform at a receiver terminal. In the DMT modulation/demodulation type transmission system, since the modulated signal is transmitted through a baseband, a resultant of a Fourier transform of the N sub-channel symbols at the transmitter should be a real value. For this, the N point IFFT at the transmitter is required to have a Hermitian symmetry characteristic of Xm,n=X*m,Nxe2x88x92n, making only N/2 QAM symbols available actually for transmission after the modulation using the N point IFFT.
Once the DMT symbol Xm=[Xm,0, Xm,1, - - - , Xm,Nxe2x88x921]T modulated at the transmitter passes a digital equivalent channel of v+1 impulse response length, it induces an ISI(Inter-Symbol Interference) to the following DMT symbol Xm+1, which can be eliminated as follows. That is, a CP(Cyclic Prefix) with a length v, i.e., Xm=[Xm,0, Xm,1, - - - , Xm,Nxe2x88x921]T is added in front of the DMT symbol xm to provide a symbol with a length N+v before transmission from the transmitter, and a guard time for giving up the first v signals is used at the receiver. In this instance, it is required at the receiver that an impulse response length of entire channel inclusive of a linear filter and a transmission line should made to be equal to, or below v+1, which is the CP length plus 1, by employing a fixed linear filter. The linear filter used in this instance is called as a TEQ(Time-domain Equalizer).
At the receiver, the CP is removed from the N+v numbers of signals passed through the TEQ, and the transmitted symbols are demodulated by using the Fourier transform, to obtain [Xm,Nxe2x88x921, Xm,Nxe2x88x922, - - - , Xm,Nxe2x88x921]T, which is in a form of multiplication of transmission signals and transmission lines, at an (n)th subchannel in an (m)th DMT symbol if there is no noise on the transmission line. The Hn denotes a response at a channel frequency xcfx89n=2xcfx80n/N, a frequency characteristic of (n)th subchannel. The subchannel frequency characteristic is by using an FEQ(Frequency-domain Equalizer), which has one complex tap that is adaptively renewed by using the LMS(Least Mean Square) algorithm, to form an inversion of a transmission function of the transmission line on the whole. By doing so, an identical determination circuit can be made available at the receiver for entire subchannels, but without any enhancement of the performance.
FIG. 2 illustrates a functional block diagram showing a system of the time-domain equalizer in FIG. 1.
As described before, in the transmission system employing the TEQ, an impulse response length of entire channel is limited to v+1 by using the TEQ, and, in order to obtain a linear filter w(D), which is the TEQ, a system as shown in FIG. 2 is used. And, once an optimal value of the linear filter is obtained by using the system, only w(D) is used in actual data transmission.
Referring to FIG. 2, it can be known that, in the DMT transmission system, a transmitter output signal at an initial stage x(D) passes through a transmission filter p(t) 21, a receiver match filter p*(xe2x88x92t) 23, and a TEQ w(D) 25, and on the same time, through an object filter b(D) 27. A subtracter 26 subtracts a signal from the TEQ 25 from the signal from the object filter 27, to provide a difference that is an error signal xe2x80x9ce(D)=x(D)b(D)xe2x88x92y(D)w(D)xe2x80x9d. The object filter b(D) is an adaptive linear filter having a (v+1) number of taps.
The error signal e(D) of the TEQ obtained thus should be minimized, by means of algorithms, such as MMSE-DFE(Minimum Mean Square Error-Decision Feedback Equalizer) method, LS(Least Square) method, Eigen Value method, and the like, and modified versions of the foregoing methods for easy implementation of the methods. When the error is minimized by using the above algorithm, an entire transmission function of the transmission filter p(t) 21, the receiver match filter p*(xe2x88x92t) 23, and the TEQ w(D) 25 has a (v+1) number of impulse responses, thereby the DMT modulated symbol Xm giving no ISI to a following symbol Xm+1. Thus, by obtaining an optimal TEQ by using a given algorithm and fixing a tap on the obtained TEQ, initialization of the TEQ is completed.
In this instance, though impulse responses of the object filter b(D) are limited to v+1, impulse responses of the TEQ w(D) 25 are not limited to v+1. Particularly, provided a number of subchannels used in the DMT transmission system increases, and a sampling frequency of a digital to analog converter and an analog to digital converter increases along with an increase of entire signal band, a length of digital equivalent impulse response of a channel having a length of time fixed physically increases. In this instance, a number of taps of the TEQ should be increased along with an increase of a size of the subchannel, for limiting impulse responses of entire channels to v+1, effectively.
The TEQ in an ADSL(Asymmetric Digital Subscriber Line) transmission system employs an FIR filter of approx. 25 taps. However, since a transmission system having maximum 4096 subchannels and a maximum sampling rate of 35.328 MHz, such as VDSL, is involved in an increase of hardware complexity as the number of subchannels increases, and has an overall operation speed increase, that impedes implementation of the TEQ by using the aforementioned method, it is required to reduce a number of taps of the TEQ, effectively.
As described, besides the problem of the hardware complexity in implementation of the TEQ in a VDSL transmission system, the VDSL transmission system has a problem of interchangeability with the ADSL transmission system. That is, the VDSL transmission system should be capable of accepting even a case when the transmitter or the receiver has the ADSL transmission system, for which the VDSL transmission system is required to maintain the TEQ system that the ADSL transmission system uses for maintaining interchangeability with the ADSL transmission system without addition of separate hardware. However, a system having the ADSL class TEQ applied to the VDSL transmission system as it is either can be used, not for a VDSL rate data transmission, but only for an ADSL rate data transmission merely, or has an operation speed of the linear filter increased for use in the VDSL rate data transmission. However, the increase of the operation speed of the linear filter for using the system in the VDSL rate data transmission is difficult to implement because, as described before, the entire impulse responses of the transmission line can not be limited to v+1 effectively due to actual limitation of the number of taps to an ADSL class even if the number of taps of the TEQ should be increased along with the increase of the number of the subchannels, and because an amount of calculation is also increased as the subchannel and the sampling rate are increased in an initializing step.
The object of the present invention designed to solve above problems in the prior art is to provide a TEQ of serial dual filter in a VDSL transmission system, in which entire signal band is divided into a low frequency band and a high frequency band, optimal values of TEQs for respective signal bands are obtained, and the two TEQs are connected in series, for carrying out a function of a TEQ for an entire signal band.
Other object of the present invention is to provide a TEQ of serial dual filter in a VDSL transmission system, in which the low frequency signal band is set identical to a band an ADSL transmission system uses, a portion of IFFT and a Fourier transform block are used in modulation and demodulation, not only for maintaining an ADSL interchangeability with easy, but also for saving hardware to save power consumption.
Another object of the present invention is to provide a TEQ of serial dual filter in a VDSL transmission system, in which an initializing is carried out after an entire signal band is divided into a plurality of signal bands, for reducing hardware required for obtaining optimal values of respective TEQs, and connecting the TEQs of different signal bands obtained in the initializing in series for obtaining an accurate frequency response of a desired band.
These and other objects and features of the present invention can be achieved by providing a method for designing TEQs(Time-domain Equalizers) for different signal bands in a VDSL transmission system, including the steps of dividing an entire signal band into at least two signal bands, and respectively modulating the divided signal bands before transmission at a transmitter, and applying respective transmitted signal bands to an algorithm that can reduce channel response lengths of the respective signal bands at a receiver, for obtaining respective TEQs.
Preferably, the entire signal band is divided into a low frequency signal band and a high frequency signal band, wherein the low frequency signal band is allocated smaller than the high frequency signal band.
More preferably, the low frequency band is over sampled, and transmitted at the transmitter, and the low frequency signal band TEQ is obtained at the receiver after down sampling the over sampled signal.
More preferably, the low frequency signal band is over sampled by xe2x80x98kxe2x80x99 times(k=N/m) (where, N/2 denotes a number of entire subchannels, and m/2 denotes a number of subchannels used) at the transmitter, and a received signal is xe2x80x98kxe2x80x99 times down sampled at the receiver.
More preferably, a number of the subchannels of the low frequency signal band is 256, and the low frequency signal band TEQ is matched with an ADSL transmission system.
In other aspect of the present invention, there is provided a device for designing TEQs for different signal bands in a VDSL transmission system, the device for dividing an entire signal band of the VDSL transmission system into at least two signal bands, and obtaining TEQs for respective signal bands, including Fourier transform blocks for modulating respective signal bands, a digital to analog converter for converting the modulated signal bands into analog and transmitting through a channel, an analog to digital converter for digitizing the analog signal transmitted through the channel, and means for applying the digital signal to an algorithm for reducing a length of a channel impulse response, for obtaining the TEQs for the different signal bands.
Preferably, the entire signal band is divided into a low frequency signal band and a high frequency signal band, wherein the low frequency signal band is allocated smaller than the high frequency signal band.
More preferably, the device further including an over sampling means for over sampling the modulated signal band for the low signal band, and providing to the digital to analog converter, and a down sampling means for down sampling the over sampled signal converted at the analog to digital converter into an original signal.
More preferably, the over sampling means over samples the signal band by xe2x80x98kxe2x80x99 times(k=N/m)(where N/2 is a number of entire subchannels, and m/2 is a number of subchannels in the low frequency band), and the down sampling means down samples a received signal by k times.
More preferably, the Fourier transform block is divided into a plurality of partial Fourier transform blocks, and the low frequency signal band is modulated by using a portion of Fourier transform blocks among the plurality of the partial Fourier transform blocks.
In another aspect of the present invention, there is provided a method for designing a TEQ in a VDSL transmission system, including the steps of dividing an entire signal band in the VDSL transmission system into at least two signal bands, obtaining the TEQs for respective signal bands, and connecting the TEQs for respective signal bands in series in a time domain.
Preferably, in the step (1), the entire signal band is divided into a high frequency signal band and a low frequency signal band, wherein the low frequency signal band is allocated smaller than the high frequency signal band.
More preferably, in the step (2), respective signal bands are applied to an algorithm for reducing a length of a channel impulse response, for obtaining the TEQs for respective signal bands.
More preferably, the low frequency signal band TEQ has a 256 subchannels, and the low frequency signal TEQ is matched with an ADSL transmission system.
In further aspect of the present invention, there is provided a TEQ in a VDSL transmission system including TEQs for different signal bands connected in series, the different signal bands being divisions of an entire signal band.
Preferably, the TEQ includes a high frequency signal band TEQ and a low frequency signal band TEQ connected in series.
More preferably, the high frequency signal band TEQ and the low frequency signal band TEQ are obtained by applying respective signal bands to an algorithm which can reduce a channel impulse response length.
More preferably, the low frequency signal band TEQ includes shift registers connected between taps for reducing data loss.